We know that sleep is important. Without it, we get tired and grumpy. We get headaches and can't think right. Our health suffers. After a number of days without sleep, we can begin seeing things or go crazy.

But we're not the only ones who need sleep. As far as we know, all animals sleep or have sleep-like behaviors. Theres a wide range. Elephants sleep about three hours; platypuses, about fourteen hours. Marine mammals, like whales, can sleep with only one side of their brain at a time. This way they can still go to the surface to breathe.

So sleep is really important. But why is it necessary? There are several theories.

Number one, the efficiency theory. Sleeping keeps us from wasting energy, so we're only active when we need to eat, that sort of thing.

Number two, the tune-up theory. When we're asleep, our bodies repair damage, eliminate poisons, produce important chemicals and all kinds of other maintenance activities.

Number three: the brain theory. While asleep, our bodies may be at rest, but our brains are busy processing new information and organizing memories.

So which one is right? Maybe all of them. Or none of them. Scientists are making a lot of progress in sleep studies. But for the moment, they'll have to sleep on it.